(1) Field of the Invention
The invention relates to a method of forming contacts in an integrated circuit device, and more particularly, to a method of forming contacts in which sidewall oxide is protected from etch damage by a nitride cap.
(2) Description of the Prior Art
In forming self-aligning contacts, the major problem area is in the contact wet etch step. Referring to FIG. 1, there is shown an enlarged view of a portion of a partially completed integrated circuit. A gate oxide layer 14 has been grown on the surface of silicon substrate 10. A polysilicon gate 16 is shown with polyoxide sidewall and overlying layer 18 which has been formed by oxidation of the polysilicon. A silicon nitride layer 20 overlies the polyoxide layer 18. A layer of tetraethoxysilane (TEOS) 22 has been deposited over the silicon nitride. Layer 23 is a photoresist layer. First a dry etch is performed to etch into the TEOS layer, which is about 3000 Angstroms in thickness, to leave about 700 Angstroms of the TEOS layer. Next, a wet etch is performed to remove the remaining TEOS. This is typically a buffered oxide etch (BOE) of 10:1 water to hydrofluoric acid. One problem that occurs with the wet etch is illustrated by 1 in FIG. 1. The photoresist is undercut into the TEOS layer. This is not a major problem. Problem area 2 is a nitride overhang caused by the isotropic etching of the polyoxide 18 under the nitride layer 20. A third problem is a stack coverage problem caused by the non-vertical polyoxide sidewall. When metal is deposited into the contact, a void may be formed because of the birds-beak formation at 3 and the metal layer will be thinner than desired because of the undercut at 2. U.S. Pat. No. 5,053,351 to Fazan et al describes a process for forming a contact opening which uses a sidewall silicon nitride cap. That process uses a single silicon nitride layer and uses a photoresist layer in forming the contact so it is not a true self-aligned contact.